The present invention relates generally to reducing the cost of electronic systems that involve voice processing, such as telephones or cellular phones, by integrating various electronics into a single silicon chip. More particularly, the present invention relates to analog-to-digital convertors that minimize a number of analog components used therein and which furthermore is relatively immune to noise pick-up from digital circuits operating in the same chip.
Many forms of conventional analog-to-digital convertors exist, for example those known by the types of techniques used therein such as Successive Approximation, Delta-Sigma Modulation, and Continuously Variable Slope Delta Modulation (CVSD). The purpose of these devices is to produce a stream of numbers representing samples of the instantaneous signal value at a desired sample rate. The desired sampling rate is usually higher than the minimum Nyquist rate of twice the maximum frequency of the analog signal to be numerically represented. The drawbacks of these prior art techniques concerns the very small signal level output from the microphone, which results in the connection between the microphone and the analog-to-digital convertor being sensitive to noise pick-up.